Electronic switch-mode DC to DC regulators convert one DC voltage level to another by storing the input energy temporarily and then releasing that energy to an output at a different voltage. The storage may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors). This conversion method is more power efficient (often 75% to 98%) than linear voltage regulation (which dissipates unwanted power as heat). The efficiency of switching regulators is beneficial to increasing the running time of battery operated devices and has increased due to the use of power FETs, which are able to switch more efficiently at high frequencies.
Switching regulators may operate in multiple operating modes depending on the current requirements of the load (“load current”). For example, to handle normal or high load current conditions, a pulse width modulation (PWM) mode is commonly used to store the input energy. In this mode, the switching regulator outputs a series of pulses having pulse width that are adjusted to store the input energy to satisfy the required load current. To handle low load current conditions, a low power mode (LPM) of operation can be used where the switching regulator outputs a series of narrow pulses where one or more of the pulses may be skipped in order to meet the reduced load current requirements. This mode is referred to as pulse frequency modulation (PFM) or PFM mode of operation. In PFM, the frequency of the pulses and the duration of a burst of pulses can be adjusted to meet the required load current.
One reason for using the PFM mode in a switching regulator is to gain light load efficiency. Typically, a threshold is set that changes the mode of operation between the PWM and PFM modes. One technique is based on sensing the peak output current. Unfortunately, the methods to determine the average load current from the peak output current can be complicated if the accuracy of the output voltage has to be maintained.
It is therefore desirable to have a simple way to transition a switching regulator between operating modes to overcome the disadvantages of complex conventional circuits, while being able to adjust the threshold or the transition point easily. A more precise solution would improve performance while an architecturally simple solution would reduce or minimize the area and cost of the design.